Aircraft Hanger grounding

[kbsparky]

I am looking at some plans/specs for a proposed aircraft hanger.

In the specs, they are calling for the installation of a static electrical grounding system, and referenced NFPA 409.

In each bay of the hanger, they are calling for (2) floor mounted grounding receptacles, where one would plug-in a ground wire and attach it to the frame of the plane to prevent build-up of static electrical charges.

They want cad-weld connections on the whole system (expected this) and a boat load of ground rods behind the hangers, all of which are interconnected and cad-welded together.

Now for the "what was that?" part:

... the bare copper wire used to connect the grounding system to the devices, in or under the slab, on grade shall be enclosed in PVC conduit.

.

HUH?? Wouldn't the use of PVC conduit kind of diminish the effect of obtaining a good ground of the bare wire in contact with the concrete and/or earth?? I would have thought that the more contact with the soil, the better ground. What am I missing here?

Have any of you had experience with aircraft hangers, and the installation of static discharge grounding systems?

 

[Pierre C Belarge]

When I read this, my mind went to pools, and equipotential bonding. The NEC permits insulated bonding conductors for pools, same for GEC.

It does not make too much sense, you may want to contact the engineer and ask him a question.

 

[jwr570]

The way I read it is, that the leads from the receptacles to the grounding system must be in pvc. Not the whole system.

 

[Dave6]

Ask the engineer to explain that one a little more. How do the plans reflect on that statment? I've done several projects with static grounding and never heard of anything like that.

 

[kbsparky]

After a closer look at the plans, the term "Counterpoise Wire" is used on the grounding grid.

Looking up that term, it would appear that counterpoise is used for radio transmitter tower grounding applications. Insulated grounding wires installed out to a certain point and then attached to driven ground rods, or something like that.

As previously stated, I have no clue as to why they would want such a system for static discharge grounding.

They did reference NFPA 409 for design information. The only thing I could find in NFPA 409 was the requirement that floor grounding outlets be installed. No mention on minimum size of wire, etc. They did mention that a maximum of 10,000 ohms would be preferable in the annex, however. That's a lot of resistance, IMO.

Any engineers here care to comment?

 

[ceb58]

After a closer look at the plans, the term "Counterpoise Wire" is used on the grounding grid.

Looking up that term, it would appear that counterpoise is used for radio transmitter tower grounding applications. Insulated grounding wires installed out to a certain point and then attached to driven ground rods, or something like that.

As previously stated, I have no clue as to why they would want such a system for static discharge grounding.

They did reference NFPA 409 for design information. The only thing I could find in NFPA 409 was the requirement that floor grounding outlets be installed. No mention on minimum size of wire, etc. They did mention that a maximum of 10,000 ohms would be preferable in the annex, however. That's a lot of resistance, IMO.

Any engineers here care to comment?

Do not know about hangers but on our communication towers we use all uninsulated tin coated copper in the ground. Trying to get as much as possible to augment the rods. We go for no more than 5 ohms.

 

[kbsparky]

Do not know about hangers but on our communication towers we use all uninsulated tin coated copper in the ground. Trying to get as much as possible to augment the rods. We go for no more than 5 ohms.

Yeah, that was my thinking as well. We replaced a main feeder cable for a radio station transmitter service, and their engineer laid an extra piece of flat copper into our trench while it was still open. It looked like about the size of a roll of toilet paper, except it was made of copper. A couple hundred feet of that in contact with the earth makes for a good supplemental ground. He installed a grounding bus at the far end, and bonded it with the meter socket at the base of the utility pole.

In the limited reading I can find on the counterpoise stuff, it would appear to be related to shaping the waveform of certain transmitter frequencies?